"It's not what we want right now," Yankees manager Joe Girardi said after New York's 6-5 win over the Orioles. "We'll keep our fingers crossed that it's not much."
Gardner struck out looking in his only at-bat against Baltimore left-hander Wei-Yin Chen, leading off the game. After the strikeout, he walked directly down the dugout steps and to the clubhouse, an indication that there was an issue of some severity.
"I've never had this kind of thing before, so I don't really know how to gauge it or what to expect," Gardner said.
Gardner was replaced in center field by Curtis Granderson, who hit a seventh-inning homer in New York's victory. Girardi said that he expected to be without Gardner's services for at least "a couple of days."
"It felt serious enough for me to say something and come out of the game, so you take what you want from that," Gardner said. "I don't think anything's broken, obviously, but it's just one of those things where I've never had an issue before.
"I know something's not right. I felt like if I went out there, it was just going to get worse and I was going to hurt the team, because I wasn't anywhere close to 100 percent."
Losing Gardner for an extended period of time could be a costly blow to the Yankees, who enter play on Friday trailing the Rays by just one game for the second American League Wild Card spot.
Gardner has been one of the Yankees' most consistent performers this year, emerging both on the field and in the clubhouse.
In 145 games, he has batted .273, setting single-season career highs in hits (147), doubles (33) and home runs (eight). Gardner has also stolen 24 bases in 32 attempts and played Gold Glove-caliber defense.
"It's frustrating. I know how important all the games are," Gardner said.
Bryan Hoch is a reporter for MLB.com. Follow him on Twitter @bryanhoch and read his MLBlog, Bombers Beat. This story was not subject to the approval of Major League Baseball or its clubs.